In piston accumulators of the above mentioned kind sealing rings are used which are pressed against the sliding surface by the pressure medium whereby very good sealing is achieved, but the friction and heat release are high. The heat release can be so high that special cooling devices for the pressure medium are needed. If on the other hand low-friction sealings are used the piston will slide easily without any considerable heat release, but the sealing effect can be insufficient especially when the piston is displaced to its initial position, i.e. when the accumulator is discharged and the piston is at the end wall of the liquid chamber.